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Zhoufang was quick to reply and this is now an option.
These SiC things aren't cheap - I may not be getting the best out of them in source-follower.
I am wondering if I'm looking at the wrong parameter - Ciss. For a cascoded source follower isn't transconductance more important ?
In this case the SiC devices is not great, neither the IRF510. But looking back to the SEWA amp again theIRFP150 has 10x the transconductance of the IRF510.
These SiC things aren't cheap - I may not be getting the best out of them in source-follower.
I am wondering if I'm looking at the wrong parameter - Ciss. For a cascoded source follower isn't transconductance more important ?
In this case the SiC devices is not great, neither the IRF510. But looking back to the SEWA amp again theIRFP150 has 10x the transconductance of the IRF510.
Maybe "sensitivity analysis" would help identify which variables affect your desired results the most.
Step 1 is perhaps the most difficult: quantify your goal. Choose an "objective function". It might be (THD20khz / Ciss). It might be (Zout/Qgd @ Id=2A). Decide what you wish to maximize and express it as a measurable number.
Step 2 is to calculate the partial derivatives: d(objective)/d(variable1) , d(objective)/d(variable2) , ... , for all the variables you are considering .
Step 3 is to identify the variable(s) with the greatest sensitivity, i.e., the largest partial derivatives. Now you know which variable affects your desired goal the most. Now you know what to look for when selecting a transistor.
You could accomplish Step 2 by hand analysis, or circuit simulation, or a combination of both. Whatever is the path of least resistance (and greatest faith or comfort) for you.
Step 1 is perhaps the most difficult: quantify your goal. Choose an "objective function". It might be (THD20khz / Ciss). It might be (Zout/Qgd @ Id=2A). Decide what you wish to maximize and express it as a measurable number.
Step 2 is to calculate the partial derivatives: d(objective)/d(variable1) , d(objective)/d(variable2) , ... , for all the variables you are considering .
Step 3 is to identify the variable(s) with the greatest sensitivity, i.e., the largest partial derivatives. Now you know which variable affects your desired goal the most. Now you know what to look for when selecting a transistor.
You could accomplish Step 2 by hand analysis, or circuit simulation, or a combination of both. Whatever is the path of least resistance (and greatest faith or comfort) for you.
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That's a good engineering approach and to a large extent I think that has been guiding my thinking. But I am also aware that the 'sound' doesn't always have a strong bearing on these factors, once they are inside a reasonable envelope. Hence the benefit of opinions from the forum based on personal experience. I guess, ultimately I can try more than one device and answer that myself 
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